One of the problems in the operation of a vehicle, particularly a heavy duty vehicle such as a motor grader, is the effect of terrain irregularities on the attitude of the vehicle and its associated adjustably connected ground engaging tools. In the past, an operator had to change the adjustment of the ground engaging tool for each roll or pitch of the vehicle; i.e., those movements not otherwise corrected by the vehicle suspension system. Some vehicle reactions to road obstacles and terrain irregularities were so severe that available ground engaging tool adjustments were undesirably untimely for adequate adjustment of the tool and efficient operation.
Suspension designers attempted to overcome this problem by employing two axles on each machine side which oscillated about a center pivoted connecting arm. This arrangement provided for a machine displacement of one-half the size of a ground surface irregularity. A performance difficulty with the above oscillating arrangement occurs during braking or high torque acceleration modes, where the braking or final drive reaction torque is on the connecting arm, the oscillating connecting arm has an undesirable tumbling force about its axis which causes excessive loading on one of a set of tandem wheels. This decreases the effectiveness of the braking action, causes uneven tire wear and imposes excessive loads on the tires.
U.S. Pat. No. 3,953,040 which issued on Apr. 27, 1976 to Dale H. Unruh and Jan B. Yates discloses a cushioning, leveling and lockup system for a suspended axle vehicle which provides for continuous load leveling and selective cushioning of road shocks. A leveling control subsystem automatically corrects any changes in the distance between the vehicle frame and axle by communicating pressurized fluid to reposition suspension cylinder struts. This system, however, repositions the load only after a preselected amount of movement of the frame relative to the axle so that transverse frame displacement is independent of the obstacle encountered. Thus, for small obstacles and ground irregularities, the frame does not level itself.
U.S. Pat. No. 3,970,327 which issued on July 20, 1976 to Joseph E. Dezelan discloses a tandem axle suspension assembly for a vehicle. This suspension system improves upon the suspension system of Unruh and Yates in that the frame is leveled for any size obstacle or ground irregularity. However, in this tandem axle suspension assembly, the frame is always displaced a distance equal to one-half the size of the obstacle or irregularity.
It is therefore desirable to have a suspension system in which the vehicle frame always moves elevationally less than one-half the height of an obstacle during movement of one of the vehicle wheels over the obstacle to thereby provide greater stability.